Today I looked in the Audi parts fiche to make sure that ALL the A4
(1.8, 2.8, 96-00) filters were the same and they are. There are no
different filters for 96-97 and 98-99.

K&N has it listed as such in
their catalog but it is WRONG. The only different filter listed in the
Audi fiche is for late model cars, they have a "cold weather" filter;
don't ask me.

I'm a tech and have been installing the same filters in
all A4s since day one. I just had to check the fiche and make sure
before I posted this info.
The ONLY correct filter is the 33-2125. If you install the 33-1675 in
any A4, you WILL have a 1/4" gap bypassing air around the filter.

33-2125 = 10" x 8 3/8"

33-1675 = 10" x 8 1/4"

If you have a 33-1675, I suggest you change it as a bypass around the
filter is NOT a good idea.